This invention relates to a current control for the heater of an exhaust gas oxygen sensor and, more particularly, to a control for maintaining a desired temperature of the oxygen sensor.
Motor vehicle exhaust gas emission controls rely almost exclusively on catalytic conversion, and the conversion efficiency is optimized through the use of air/fuel ratio feedback signals developed by exhaust gas oxygen sensors. Since the oxygen sensors only operate in a warmed-up state, it is customary to package the sensors with an integral heater element that is electrically activated following engine start-up to quickly heat up the oxygen sensor and maintain it at or above a desired operating temperature, such as 600xc2x0 C. However, it is difficult to accurately determine if the oxygen sensor is sufficiently heated without actually measuring the sensor temperature, which would significantly increase the system cost. In some current production vehicles, the engine controller estimates the oxygen sensor temperature by table look-up based on various known system parameters, but such an approach requires a considerable calibration effort, and fails to take into account that the heater element electrical resistance can vary substantially from sensor to sensor. Accordingly, what is needed is an improved method of activating the heater element of an exhaust gas oxygen sensor to ensure accurate and reliable control of the sensor temperature.
The present invention is directed to an improved control for the heater element of a motor vehicle engine exhaust gas oxygen sensor, wherein the temperature of the heater element is accurately and inexpensively determined, and a closed-loop control of the heater element current minimizes deviations of the heater element temperature from a desired heater temperature determined in relation to the desired oxygen sensor temperature. In a preferred embodiment, the heater temperature is calculated based on the heater element resistance, and the heater element resistance is adaptively adjusted to account for sensor-to-sensor variation. The adaptive adjustment is updated based on a deviation of the measured heater element resistance from an expected value under predetermined calibration conditions at engine start-up.